1. Field of the Invention
The present invention relates to a method for manufacturing a solid electrolytic capacitor having a porous anode body made of powder of valve metal or an alloy thereof.
2. Description of Related Art
Solid electrolytic capacitors generally have an anode body, a dielectric layer, an electrolyte layer, and a cathode layer which are formed as follows. The anode body is formed by sintering valve metal powder such as niobium (Nb) or tantalum (Ta). The dielectric layer is formed on the anode body by anodizing its surface. The electrolyte layer and the cathode layer are formed in that order on the dielectric layer.
In recent years, most electrolyte layers are made of conductive polymers because of their high conductivity and their ability to reduce an equivalent series resistance (ESR).
One proposed conductive polymer layer used as the electrolyte layer of a solid electrolytic capacitor has a laminated structure of first and second conductive polymer layers of polypyrrole. The first and second conductive polymer layers are formed by chemical polymerization and by electrolytic polymerization, respectively (see, for example, Japanese Patent Unexamined Publication Nos. 2006-32530 and H11-191518).
In the case of chemical polymerization, a conductive polymer layer is generally formed as follows. An anode body having a dielectric layer is soaked in an oxidizing agent solution, and then is either brought into contact with the vapor of a conductive-polymer monomer or soaked in a solution of the monomer, thereby polymerizing the monomer. In the case of electrolytic polymerization, on the other hand, a conductive polymer layer is formed by applying voltage in a monomer solution to the first conductive polymer layer as an electrode, which has been formed by chemical polymerization or other methods.
Japanese Patent Unexamined Publication No. 2005-322917 proposes soaking an anode body having a dielectric layer in a mixture solution containing a conductive-polymer monomer and an oxidizing agent, and polymerizing the monomer by chemical polymerization.
Japanese Patent Unexamined Publication No. H11-26310 proposes alternate application of liquid-phase chemical polymerization and gas-phase chemical polymerization to an anode body having a dielectric layer so as to form a conductive polymer layer.
In terms of increasing the capacitance of a solid electrolytic capacitor, it is preferable that the anode body has a large surface area. To achieve a large surface area, attempts have been made to make the valve metal powder used for the anode body as fine as, for example, having a CV value of not less than 120,000 μF·V/g. The CV value is the product of the electrolysis voltage and the capacity of the anode body having a dielectric layer. In the above-mentioned conventional techniques, however, in the case of using such fine metal powder, the conductive polymer layer cannot be densely formed in small spaces or pores of the anode body. As a result, it is impossible to manufacture a solid electrolytic capacitor having a large capacitance.